The present invention relates to the art of earth boring in general and, more particularly, to a method of constructing a rotary rock bit and a system for equalizing pressures in the lubrication system of a rotary rock bit. Problems have been encountered with prior art systems of supplying lubricant to the bearings of a rotary rock bit because of the relatively long lifetime of modern bits and the wide variation of environmental conditions encountered during the drilling operation. When a rotary rock bit is lowered into a well bore, the environmental pressure surrounding the bit increases at the rate of approximately 1/2 pound per square inch for each foot of depth. This means that at a depth of 10,000 feet the hydrostatic pressure on the outside of the bit could be 5,000 p.s.i. or more because of the weight of the drilling fluid in the well bore above the bit. In order for a lubrication system to function properly at the elevated downhole pressures, some means must be provided to equalize the internal pressure of the lubricant in the lubrication system with the hydrostatic pressure of the drilling fluid in the well bore. Failure to provide an effective pressure equalizing system in prior art rock bits has resulted in the destruction of one or more elements of the lubrication system. The elements of the lubrication system that have been the most susceptible to destruction in the prior art rock bits are the seal and the flexible diaphragm in the lubricant reservoir.
A wide variety of other environmental conditions also affect the performance of a pressure equalizing system. For example, the temperature will rise as the well bore penetrates deeper into the earth and temperatures in the range of 250.degree. to 350.degree. at 10,000 foot depth may be expected with even higher temperatures at greater depths. As the bit is rotated and the cone cutters engage the formations, a large amount of heat is generated causing the environmental temperature of the bit to rise. The elevated temperature has an adverse effect on the lubricant, the structural elements of the bit including the lubrication system, the pressure equalizing system and the bearings.
Vacillating pressure conditions, including the magnitude of vacillation, must also be considered when providing a pressure equalizing system. Periodic pressure variations are produced during the drilling operation and these pressure variations can damage the structural elements of the lubrication system and the pressure equalizing system. During drilling, joints of pipe must be added to the drill string for progressively deeper penetration. This may mean that 50 or 60 joints of pipe are added to the drill string during the normal life of a sealed bearing rotary rock bit. In order to add a joint of pipe, which is usually 30 feet in length, rotation of the bit must be stopped and the entire string of pipe, including the bit, must be raised high enough to allow the kelly to clear the rotary table (35-50 feet). Since operating costs of an oil well drilling rig are quite high, the time that the bit is off bottom and not drilling must be kept to a minimum. Therefore, the addition of a joint of pipe must be accomplished quickly and the drill string must be raised and lowered as rapidly as possible. This raising and lowering of the drill string creates pressure variations that affect the lubrication system and the pressure equalizing system.
When the bit is on bottom, the pressure of lubricant is the same as, or nearly the same as, the hydrostatic pressure of fluid in the well bore. However, as the drill string is elevated in the well bore, the bit body acts in much the same manner as a piston in a cylinder. The enlarged diameter of the bit body exerts a force on the column of fluid above it due to the velocity of the bit traveling up the well bore. The velocity of the fluid moving past the large diameter portion of the bit may be fairly high causing a low pressure area in the zone between the cutters and the main bit body where the seal is located. The pressure differential between the pressure of fluid in the area of the seal and the pressure of the lubricant inside of the bit may be in the order of 100 p.s.i. or more during periods of high acceleration of the drill string.
In a substantial number of sealed bearing rotary rock bits, seals are used that resist flow in both directions. An example of this type of seal is an O-ring seal. Bits using this type of seal may encounter a substantial pressure build-up within the lubrication system. Some of the potential sources of the pressure build-up are the pressure differential between lubricant inside of the bit and the fluid in the well bore outside of the bit and thermal expansion of the lubricant caused by the elevated temperatures encountered during the drilling operation.